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www.usra.edu
X-Ray Photoelectron Spectroscopy (XPS)
Applied to Soot &
What It Can Do for You
Randy L. Vander Wal, Vicki Bryg & Michael D. Hays USRA The U.S. EPA
Acknowledgements: 1. Former funding through NASA NRA 99-HEDs-01 (RVW) and the U.S. EPA 2. Soot samples supplied by the U.S. EPA, Sandia National Labs 3. Vicki Bryg, Patrick Rodgers, Y.L.Chen, David R. Hull and Dr. Chuck
Mueller, Sandia National Labs
Results Regarding Soot Nanostructure
Soot Nanostructure: (Definition) * Soot Nanostructure refers to carbon lamella (layer plane) length, orientation, separation and tortuosity. * Nanostructure is variable, dependent upon temperature, residence time and fuel identity.
Fringe Analysis Algorithm: (Quantification) * Lattice fringe analysis can be used to analyze HRTEM image data and quantify carbon nanostructure through statistical analysis.
Oxidation Rates: (Implications) * Oxidation rates are dependent upon nanostructure - suggests using nanostructure to control (accelerate) oxidation. * Source apportionment via analysis of nanostructure? * Health consequences related to nanostructure? * Environmental impact dependent upon nanostructure?
Pure HydrocarbonApplication I: Emissions Reduction
Oxygenated Additive
Fringe Length Analysis
Reference Fuel-n-hexadecane + heptamethylnonane 25
20
15
10
5
00.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76
Fringe Length (nm)
Diethylene glycol ether (DGE)25
20
15
10
5
00.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76
Fringe Length (nm)
Fringe Tortuosity Analysis
Reference Fuel- n-hexadecane + heptamethylnonane 20
15
10
5
01.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96
Fringe Tortuosity
Diethylene glycol ether (DGE)20
15
10
5
01.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96
Fringe Tortuosity
Smoke Meter (Engine Out)
Microscopic and Spectroscopic Analysis Techniques for Soot Characterization
HRTEMHRTEM(high resolution transmission(high resolution transmissionelectron microscopy)electron microscopy)
Microscopy TechniqueMicroscopy Technique
Physical StructurePhysical Structure(nanostructure)(nanostructure)
XPS (X-ray photoelectron spectroscopy)
Spectroscopy Technique
Chemical Composition (& bonding states)
Outline - XPS
1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:
A. Elemental Composition (Identification of source; wear, etc.)
B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)
* Consistency of samples within the same class * Distinctness between different classes of samples
C. Carbon (nano)structure
4. Conclusions
Introduction to X-Ray Photoelectron Spectroscopy (XPS)
* XPS provides information about elemental composition and oxidation state of the surface.
* A monochromatic X-ray beam of known energy displaces an electron from a K-shell orbital.
* The kinetic energy of the emitted electron is measured in an electron spectrometer.
* The binding energy Eb = hv – Ek is characteristic of the atom and orbital from which the electron is emitted.
Introduction to XPS (continued)
* A low-resolution wide-scan (survey) spectrum serves as the basis for the determination of the elemental composition of samples.
* At higher resolution, chemical shifts are observed depending upon oxidation state.
Ek = hv - Eb
X-ray e-
Eb
Outline
1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:
A. Elemental Composition (Identification of source; wear, etc.)
B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)
* Consistency of samples within the same class * Distinctness between different classes of samples
C. Carbon (nano)structure
4. Conclusions
Low-Resolution Survey Scan - Jet Aircraft Emissions
Low-Resolution Survey Scan - Diesel Engine Soot
6.0
5.0
4.0
% 3.0
2.0
1.0
0.0
XPS - Survey Elemental Analysis
S
Plant OFB
Wildfire
Jet Engines
Industrial OFB
Biodiesel A
Residential OFF
Biodiesel B
Biodiesel Soot
Na N Ca P
Elements
Outline
1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:
A. Elemental Composition (Identification of source; wear, etc.)
B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)
* Consistency of samples within the same class * Distinctness between different classes of samples
C. Carbon (nano)structure
4. Conclusions
Identify Oxygen Functional Groups
C-C sp2
Carbonyls
Phenols
Plasmon Carboxylic
-C-OH Phenol -C=O Carbonyl
-C-OOH Carboxylic
% A
rea
Oil Fired Boiler versus Residential Oil Fired Furnace
% A
rea
50
40
30
20
10
0
C1s Peak Histogram 80
C1s Peak Histogram
C-C sp2 60
C-C sp2
Fullerenic
40
-C-OH
C-C sp3
-C=O
0
20
-C-OOH -C-OOH
C-C sp3
-C=O
Industrial OFB1 Industrial OFB2
Res. OFF1 Res. OFF2
281.6 283.2 284.4 285.2 286.3 288.2 289.5 281.6 283.2 284.4 285.2 286.3 288.2 289.5
C1s Peak Position (eV) C1s Peak Position (eV)
0
10
20
30
40
50
60
% A
rea
Comparison Between Biodiesel Soots
C1s Peak Histogram
Biodiesel 15&16 Biodiesel 17&18
0
20
40
60
80
100 C1s Peak Histogram
% A
rea
C-C sp2
-C-OH -C-OOH
C-C sp2
-C-OH
-C=O
Fullerenic
Biodiesel 13Biodiesel 14
281.6 283.2 284.4 285.2 286.3 288.2 289.5 282.8 283.6 284.8 285.8 287 288.8
C1s Peak Position (eV) C1s Peak Position (eV)
Comparative Peak Intensities - Oxygen Functional GroupsAverage C1s Peaks
(normalized)
Diesel Soot
Plant OFB
Wildfire%
Jet Engine%
Industrial OFB
Residential OFF
Biodiesel A
Biodiesel B
281.4
283.4
284.2
284.7
285.0
285.9
286.3
C1s position (eV) 286.6
287.4
288.8
289.5
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Outline
1. Motivation & Background 2. Introduction to XPS 3. Analytical capabilities:
A. Elemental Composition (Identification of source; wear, etc.)
B. Carbon Oxidation State - Oxygen Functional Groups (Oxidation conditions)
* Consistency of samples within the same class * Distinctness between different classes of samples
C. Carbon (nano)structure
4. Conclusions
Motivation for Alternative Analysis Techniques
1. Lattice Fringe Analysis is time consuming
2. Analysis can be difficult to apply (in some cases)
Hmmm….
Identify Types of Carbon Nanostructure
C-C sp2
Carbonyls Phenols
Fullerenic
Carboxylic
C-C sp3
Identify Types of Carbon Nanostructure
Carbonyls Phenols
Carboxylic
C-C sp3 C-C sp2
Fullerenic
Identify Types of Carbon Nanostructure
C-C sp2
Carbonyls
Phenols
Fullerenic
Carboxylic
C-C sp3
Ave
rage
Inte
nsity
(raw
dat
a)
281.
4
283.
1
283.
6
284.
2
284.
4
284.
6
285.
0
285.
7
286.
0
286.
3
286.
5
286.
8
287.
4
287.
9
288.
8
289.
5
Comparative Peak Intensities - Carbon Nanostructure12000
Average XPS peaks
10000
Diesel Soot Plant OFB
8000 Wildfire Jet engines Industrial OFB6000 Residential OFF Biodiesel A
4000 Biodiesel B
2000
0
Peak positions
High Resolution Scan - C1s Region - HOPG Edge Planes
C-C sp2
C-C sp3
Carboxylic
Phenols
Carbonyl
Graphitic
Defects
High Resolution Scan - C1s Region - HOPG Layer Planes
C-C sp2
C-C sp3
Phenols
Graphitic
Defects
XPS Sensitivity to Carbon Nanostructure
C-C sp2
C-C sp3
Phenols
C-C sp2
C-C sp3
Carboxylic
Phenols
Carbonyl
* Developing Correlations with Carbon Reactivity*
Sample: Edge Sites Basal Plane Intensity Sum Intensity Sum
Planar Graphite ( ~ 284 eV, sp2)
1749 13%
11265 87%
Edge Graphite ( ~ 285 eV, sp3)
4021 37%
6723 63%
Ratio (G/D) 0.35 1.38
Edge site carbons can be nearly 10-fold more reactive than basal plane sites
Goal & Result
HRTEM & Lattice Fringe Analysis
XPS Carbon nanostructure Oxidative Reactivity
Conclusions
1. XPS analysis can identify & quantify trace elements. Utility: Can be used to identify source based on specific elements
present and their distribution. * Track fuel and/or oil elements * Analysis of engine wear
2. XPS can identify oxygen groups by bonding type; C-OH, C=O, and C-OOH. These reflect the soot oxidation history.
Utility: Identify the occurrence and degree of oxidation, such as the soot cake within a DPF
3. XPS can identify the types of carbon present, sp2, sp3 and fullerenic Therein it can provide a complimentary method to HRTEM and image analysis.
Utility: Correlate nanostructure with soot reactivity, and changes new fuels, e.g. biodiesel
Graphitic
Normal Particle
DPF Soot
Hollow Interiors
outer shell
Normal Particle Normal Particle
Partially Oxidized Partially Oxidized
Application III: DPFs
Application II: Source Specific NanostructureWildfire Emissions Oil Fired Boiler
Comparison between carbon lamella;short, disconnected versus longer range structure and order
Jet Aircraft Engine Exhaust
FullerenicStructure
Jet Engine Emission Diesel Emission Wildfire Emissiona1-1c-ROI_14 e2-3a-roi_1c4-hb-ROI_1 252525
202020
151515
101010
555
000 0.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76 0 0.72 1.44 2.16 2.88 3.6 4.32 5.04 5.76 0.00 0.72 1.44 2.16 2.88 3.60 4.32 5.04 5.76
Fringe Length (nm) Fringe Length (nm) Fringe Length (nm)
C4-HB-SEPSKEL_1 A1-1C-SEPSKEL_1 E2-3A-ROI_1_4 25 25
25
202020
151515
101010
555
000 0.32 0.35 0.37 0.40 0.43 0.46 0.48 0.32 0.35 0.37 0.40 0.43 0.46 0.48 0.32 0.35 0.37 0.40 0.43 0.46 0.48
Fringe Separation (nm) Separation Distance (nm) Separation Distance (nm)Separation Distance (nm)
c4-hb-tort_1 a1-1c-tort_1 e2-3a-_1 202020
151515
101010
555
000 1.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96 1.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96 1.00 1.12 1.24 1.36 1.48 1.60 1.72 1.84 1.96
Tortuosity Tortuosity Tortuosity
Average C1s Peaks Average C1s PeaksIntensitiies and Locations Intensitiies and Locations
Ave
rage
Inte
nsity
2500
2000
1500
1000
500
0 0
500
1000
1500
2000
281.4 283.6 284.4 285.2 286.4 287 288.8
Bio 13-18
Ave
rage
Inte
nsity
Average Peak Position (eV)
281.4 283.6 284.4 285.2 286.4 287 288.8
Bio 13-14
Average Peak Position
0
2000
4000
6000
8000
1 104
Average C1s PeaksIntensitiies and Locations
BBB2&WVU
Ave
rage
Inte
nsity
Biodiesel 1 Biodiesel 2
Ordinary diesel
Averaged by Intensity
Averaged by Position
281.4 283.6 284.4 285.2 286.4 287 288.8
Average Peak Position (eV)